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Commit d2aa1aca authored by Kees Cook's avatar Kees Cook Committed by Ingo Molnar
Browse files

mm/init: Add 'rodata=off' boot cmdline parameter to disable read-only kernel mappings 



It may be useful to debug writes to the readonly sections of memory,
so provide a cmdline "rodata=off" to allow for this. This can be
expanded in the future to support "log" and "write" modes, but that
will need to be architecture-specific.

This also makes KDB software breakpoints more usable, as read-only
mappings can now be disabled on any kernel.

Suggested-by: default avatarH. Peter Anvin <hpa@zytor.com>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Brown <david.brown@linaro.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-hardening@lists.openwall.com
Cc: linux-arch <linux-arch@vger.kernel.org>
Link: http://lkml.kernel.org/r/1455748879-21872-3-git-send-email-keescook@chromium.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e267d97b

Documentation/kernel-parameters.txt

+4 −0
Original line number Diff line number Diff line
@@ -3491,6 +3491,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted. 


	ro		[KNL] Mount root device read-only on boot



	rodata=		[KNL]

		on	Mark read-only kernel memory as read-only (default).

		off	Leave read-only kernel memory writable for debugging.



	root=		[KNL] Root filesystem

			See name_to_dev_t comment in init/do_mounts.c.

init/main.c

+23 −4
Original line number Diff line number Diff line
@@ -93,9 +93,6 @@ static int kernel_init(void *); 
extern void init_IRQ(void);

extern void fork_init(void);

extern void radix_tree_init(void);

#ifndef CONFIG_DEBUG_RODATA

static inline void mark_rodata_ro(void) { }

#endif



/*

 * Debug helper: via this flag we know that we are in 'early bootup code'
@@ -929,6 +926,28 @@ static int try_to_run_init_process(const char *init_filename) 


static noinline void __init kernel_init_freeable(void);



#ifdef CONFIG_DEBUG_RODATA

static bool rodata_enabled = true;

static int __init set_debug_rodata(char *str)

{

	return strtobool(str, &rodata_enabled);

}

__setup("rodata=", set_debug_rodata);



static void mark_readonly(void)

{

	if (rodata_enabled)

		mark_rodata_ro();

	else

		pr_info("Kernel memory protection disabled.\n");

}

#else

static inline void mark_readonly(void)

{

	pr_warn("This architecture does not have kernel memory protection.\n");

}

#endif



static int __ref kernel_init(void *unused)

{

	int ret;
@@ -937,7 +956,7 @@ static int __ref kernel_init(void *unused) 
	/* need to finish all async __init code before freeing the memory */

	async_synchronize_full();

	free_initmem();

	mark_rodata_ro();

	mark_readonly();

	system_state = SYSTEM_RUNNING;

	numa_default_policy();

kernel/debug/kdb/kdb_bp.c

+1 −3
Original line number Diff line number Diff line
@@ -153,13 +153,11 @@ static int _kdb_bp_install(struct pt_regs *regs, kdb_bp_t *bp) 
	} else {

		kdb_printf("%s: failed to set breakpoint at 0x%lx\n",

			   __func__, bp->bp_addr);

#ifdef CONFIG_DEBUG_RODATA

		if (!bp->bp_type) {

			kdb_printf("Software breakpoints are unavailable.\n"

				   "  Change the kernel CONFIG_DEBUG_RODATA=n\n"

				   "  Boot the kernel with rodata=off\n"

				   "  OR use hw breaks: help bph\n");

		}

#endif

		return 1;

	}

	return 0;